The present invention relates to a method for making a multiphase hot-rolled steel strip having improved mechanical properties, in particular high strength and good ductility. Currently, such strips have a thickness of between 0.7 mm and 10 mm and more often between 2 mm and 6 mm.
High-strength steels have been known for a long time in the prior art and they have many different uses. In many cases, the mechanical properties of these steels result from appropriate thermal treatment, allowing in many cases to avoid having recourse to alloying elements, which are generally expensive.
However, certain applications require hot-rolled steel strips that have both high strength and good forming properties. Currently, such a combination of properties is extremely difficult to achieve and moreover is generally obtained only by means of multiphase steels such as steels with a ferrite/bainite or ferrite/martensite microstructure or by three-phase steels. In these steels, the ferrite forms the ductile and deformable element, while the second phase, bainite or martensite, strengthens the steel. The final mechanical properties of the steel are directly affected by the respective proportions of these phases and by the temperatures at which these are formed.
According to conventional practice, steels with a ferrite/bainite or ferrite/martensite microstructure are obtained from a specific chemical composition and by strict control of the cooling conditions during hot rolling. The microstructure and properties of these steels are affected by the coiling temperature and by the cooling rates to which the steels are subjected.
On a conventional laminar cooling table, it is not possible to control the cooling rate of the hot-rolled strip because the specific delivery rates of the cooling liquid are fixed. This cooling rate will therefore largely depend on the speed and thickness of the strip and on external parameters such as the temperature of the cooling liquid. In particular, it varies over the length of the strip owing to the increase in the speed of the latter due to the acceleration of the rolling mill between the beginning and the end of a strip. As is known, this acceleration is imposed by the need to maintain a constant end-of-roll temperature for the entire strip. This results in uncertainty as to the cooling rate of the steel, which has repercussions for the microstructure and hence properties of the strip and may ultimately be translated into costly strip cropping and degradation.
Moreover, the chemical composition of the steel must be adapted as a function of the microstructures to be achieved and likewise as a function of the cooling which might be applied. In these conditions, it is virtually impossible to vary the composition of the steel in a specific way in order to improve certain mechanical properties, such as fatigue resistance or resistance to ageing, capacity for hole expansion, or indeed suitability for welding or surface quality.
It is furthermore known that it is possible to produce multiphase steels by a cooling treatment referred to as interrupted-cycle treatment. In general terms, such treatment initially comprises a first step, in which the strip is maintained at a high temperature to ensure partial transformation of the austenite into ferrite, followed by abrupt cooling intended to solidify the partially transformed microstructure, and finally a second step, in which the temperature is maintained at a lower level to transform the rest of the austenite into bainite or into martensite. In conventional strip mills, the cooling tables do not however have cooling sections that are powerful enough to ensure abrupt cooling of this kind.
In this regard, an ultra-fast cooling method (UFC) is indeed known, applied to a hot-rolled strip immediately after it emerges from the finishing mill. This ultra-fast cooling is followed by slow cooling, referred to as laminar cooling, on the conventional cooler leading to the coilers. This method does, of course, allow to obtain steels with a high elastic limit, e.g. steels containing dispersoids. However, such steels have a lower ductility than that developed by multiphase structures, preventing them from being used for applications that require one or more forming operations.
The present invention aims to propose a method for making a multiphase hot-rolled steel strip which has mechanical properties, in particular strength and ductility, that are improved compared to the above-mentioned prior art.
According to the present invention, a method for making a multiphase hot-rolled steel strip, which comprises an ultra-fast cooling operation, is characterised in that said ultra-fast cooling operation is carried out after slow laminar cooling of the strip on the cooling table and before the final coiling of the strip.
In hot-strip mills, the end-of-roll temperature of the strips is equal to or greater than the Ar3 transformation temperature; of course, this temperature varies as a function of the composition of the steel but it is generally between about 800xc2x0 C. and 900xc2x0 C.
According to the invention, the hot-rolled steel strip is subjected, on emerging from the finishing mill, to a first slow cooling operation from the end-of-roll temperature to a temperature referred to as the intermediate temperature, between about 750xc2x0 C. and 500xc2x0 C., preferably between 750xc2x0 C. and 600xc2x0 C., then to an ultra-fast cooling operation from said intermediate temperature to a temperature referred to as the coiling temperature, between about 600xc2x0 C. and room temperature, and finally to a second slow cooling operation from said coiling temperature to room temperature.
The first cooling operation preferably takes place on the conventional laminar cooling table, i.e. with water at a low cooling rate; however, it can also be carried out with air. It thus forms the first step in which the strip is maintained at a high temperature, during which the ferrite can form in conditions close to equilibrium. The duration of this first cooling operation depends on the speed of the strip and on the cooling rate applied, as a function of the degree of transformation desired and hence of the intermediate temperature intended. The cooling rate being low in all cases, it is not influenced to any significant extent by the effect of the acceleration of the mill.
The abrupt cooling operation is then preferably carried out by the ultra-fast cooling method mentioned above. It may be recalled here that this ultra-fast cooling consists in spraying the strip with jets of water under a pressure of 4 to 5 bar; this cooling can be regulated in terms of cooling rate and temperature by means of the water delivery rate and the length sprayed. It allows to achieve cooling rates of 5 to 10 times greater than conventional laminar cooling tables. Said ultra-fast cooling operation is preferably carried out at a cooling rate such that the product of the thickness of the strip in mm and the cooling rate in xc2x0 C./s is greater than 600, and preferably greater than 800. By way of illustration, the ultra-fast cooling operation mentioned above is advantageously carried out at a cooling rate greater than 150xc2x0 C./s on a 4-mm thick strip.
Finally, the second slow cooling operation is carried out immediately after the abrupt cooling operation, i.e. essentially during the coiling of the strip. This cooling operation takes place from the coiling temperature to a temperature at which there is no more transformation of the microstructure, i.e. in practice to room temperature. In the course of this slow cooling operation, the residual austenite is generally transformed to form the second phase, bainite or martensite, as a function of the coiling temperature. However, in certain cases, this transformation may take place before the slow cooling operation, i.e. during the abrupt cooling operation.
For the practical implementation of the invention, the respective proportions of the phases required in the steel are first of all determined as a function of the desired properties; the duration of the first slow cooling operation and the intermediate temperature leading to the required fraction of the first phase are deduced therefrom; the coiling temperature leading to the required second phase is likewise deduced therefrom; finally, said values for duration and temperature are applied for the respective regulation of the first slow cooling and the ultra-fast cooling stages.